Photomultiplier having wall coating of electron emitting material and photoconductive material



M r h 1969 J. R. DAVY ET AL 3,432,668 PHOTOMULTIPLIER HAVING WALL COATING OF ELECTRON EMITTING MATERIAL AND PHOTOCONDUCTIVE MATERIAL Sheet Filed Oct. 14, 1966 Inventors JOHN R DAvv, JOHN M.BALLANTINE f: WILLlAM a. ALLAN Attorneys March 11, 1969 J. R. DAVY ET AL 3,432,668 CTRON EMITTING PHOTOMULTIPLIER HAVING WALL bOATING OF BLE AND PHOTOCONDUCTIVE MATERIAL MATERIAL Filed 001',- 14, 1966 Sheet 3 of2 I Inventors M L r L w A? n w SW DI zu h Moms. T m

JOHN M. BALLANT'INE Was n/L 32 w United States Patent 43,912/ 65 U.S. Cl. 250-207 Int. Cl. H0lj 39/12 9 Claims ABSTRACT OF THE DISCLOSURE An electron multiplier having varying amplification including a tube provided on the inner surface with a coat ing of electron-emitting material with the input end of the tube being provided with a second layer of photoconductive material so that amplification of an electron stream passing through the tube is dependent upon the amount of light striking the photoconductive material.

Image intensifiers have been proposed, in which an image is focussed on to a film of material which emits electrons when irradiated by photons, above a certain energy. The material is basically a mixture of alkali metals. If the incident light is below the critical wavelength, then electrons are emitted from a point on the film in quantities which are proportional to the image intensity at that point. Thus the optical image is converted into an electron object.

The electrons are focussed, either electrostatically or magnetically, onto a phosphor screen and are accelerated towards this screen. The electrons impinge on the phosphor and cause light to be emitted. The image thus appears on the phosphor screen and its brightness is determined by the accelerating voltage applied to the electrons. The phosphor emits light in the visible spectrum but the alkali screen can be made sensitive to light of wavelengths up to 1.2, approx. This means that an image in the near infrared can be converted to a visible image.

An alternative method of increasing the signal from a photon is employed in a photomultiplier. In this the photon emits an electron from an alkali film, as above. The electron is accelerated and strikes a dynode where secondary electrons are emitted. These secondaries are accelerated onto a second dynode where more secondaries are ejected. The original electron thus gives rise to a large number of secondary electrons, the number depending on the number and type of dynodes, and the accelerating voltages. The electron multiplication section is a complicated set of metal structures fed by a chain resistor network.

It is an object of the present invention to provide an improved photomultiplier for use in image intensifiers or converters.

The present invention is an electron multiplier comprising a tube having, on the inner surface thereof, a first layer of secondary electron-emitting, electrically conducting material and, adjacent one end, the input end, a second layer of photoconductive material, terminal means being provided for connecting a source of voltage along the tube and in series with said layers whereby the amplification of an electron stream passing down the tube may be made dependent on the amount of light striking the layer of photoconductive material.

The present invention isalso an image intensifier or converter comprising an evacuated tubular shell having at one end an input window and at the other end detector means, at least one electron multiplier as defined 3,432,668 Patented Mar. 11, 1969 in the last preceding paragraph mounted intermediate the ends of the tube, first means for providing a background stream of electrons mounted in the tube adjacent the window and second means for maintaining the first means at the same potential as the input end of the electron multiplier.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of an image intensifier according to the present invention, part of the outer shell of the image intensifier being broken away to show the internal components; and

FIG. 2 is a detailed, enlarged view of an electron multiplier as used in the image intensifier of FIG. 1.

Referring now to FIG. 1, an image intensifier 10 according to the present invention consists of an evacuated tubular shell 11 having an input window 12 at one end and detector means in the form of a phosphor screen 13 at the other end.

A long half-life ,9 emitter in the form of a ring 14 is provided immediately behind the input window 12 to pro-v vide a background stream of electrons. A bundle 15 of electron multipliers 15, one of which is illustrated in FIG. 2, is also mounted in the shell 11 and terminates a short distance from the phosphor screen 13. Three voltage leads 17 are provided, one to each end of the bundle 15 and one to the phosphor screen 13, to be connected to a voltage source 18 as shown. The inner surface of the shell 11 is provided, between ring 14 and the input end of the bundle 15, with a metal coating 19 to maintain the ring 14 at the same electrical potential as the input end of the bundle, i.e. the left-hand end of FIG. 1.

An electron multiplier 16 from the bundle 15 as illustrated in FIG. 2 to a greatly enlarged scale as in practice each multiplier has a diameter small enough to be below the resolution of a detecting device, e.g. a human eye, at the phosphor screen. Each electron multiplier 16 consists of a glass tube 24 coated on its inner surface with a secondary electron emitter electrically conducting material 25, in this embodiment silicon. Adjacent the input end of the electron multiplier, i.e. the left hand in FIG. 2 is provided a second layer 26 of photoconductor material. In this embodiment the layer 26 is of lead sulphide and, as the input window should be made from a material transparent through the sensitive region of the photoconductor, the input window 12 is made of sapphire or calcium acetate.

The ends of the electron multipliers are rendered conducting by evapourating on to them a suitable metal 28, e.g. aluminium, to allow for connection of the ends of the bundle 15 to the appropriate voltage leads 17'.

In use considering firstly a single electron multiplier 16 in the image intensifier 10, assuming no incident light is present, electrons from the ,8 emitter ring 14 entering the tube at the input end are accelerated along the tube due to the voltage differential, strike the secondary emitter and cause the emission of more secondary electrons thus amplifying the original beam. The degree of amplification is, however, critically dependent upon the voltage differential across the ends of the electron emitter coating 25 and, in the absence of incident light is so small that the resultant intensity on the phosphor screen 13 would be low. If, however, light within the sensitive range of the photo-conductor layer 26 is incident into the tube and onto the photo-conductor layer 26, the voltage across the electron emitter coating 25 will vary according to the intensity of this light and therefore the amplification of the electron beam will vary with the intensity of the light. The increase in voltage across the coating 25 is not linear with light input, but the electron gain provided by the coating 25 is not linear with applied voltage and it is possible to choose the parameters such that the gain of the coating 25 changes linearly with alteration in light input. The phosphor screen 13 of course, converts the electron beam into visible light and its output intensity also varies in sympathy with the intensity of the incident light. By providing a bundle 15 of electron multipliers 16 as described a coherent picture rather than merely a signal may be provided on the screen 13.

Using the present invention image converters or intensifiers may be made that are sensitive over the wavelength range of photo-conductors. This frees such devices from the stigma of very limited sensitivity of the electron emitter materials in the 1 micron range.

What we claim is:

1. An electron multiplier comprising a hollow tube having an inner surface and an input end and an output end, a first layer of secondary electron-emitting electrically conducting material on said inner surface, a second layer of photoconductive material adjacent said input end, terminal means connecting a source of voltage along the tube and in series with said layers, so that the amplification of an electron stream passing down the tube is dependent on the amount of light striking the layer of photo-conductive material.

2. An electron multiplier as claimed in claim 1, in which the secondary electron emitting material is silicon.

3. An electron multiplier as claimed in claim 1, in which the photoconductive material is lead sulphide.

4. An image intensifier or converter comprising an evacuated tubular shell having at one end an input window and at the other end detector means, at least one electron multiplifier comprising a hollow tube haivng an inner surface and an input end and an output end, a first layer of secondary electron-emitting electrically conductive material on said inner surface, a second layer of photoconductive material adjacent said input end, terminal means connecting a source of voltage along the tube and in series with said so that the amplification of an electron stream passing down the tube is dependent upon the amount of light striking the layer of photoconductive material, said electron multiplier being mounted intermediate the ends of the tubular shell, first means for providing a background stream of electrons mounted in the tubular shell adjacent the window and second means for maintaining the first means at the same potential as the input end of the electron multiplier.

5. An image intensifier or converter as claimed in claim 4, in which said detector means consists of a phosphor screen.

6. An image intensifier or converter as claimed in claim 5, wherein a bundle of electron multipliers is employed.

7. An image intensifier or converter as claimed in claim 6, in which the diameter of each multiplier in the bundle is small enough to be below the resolution of the detecting means.

8. An image intensifier or converter as claimed in claim 7, in which said first means is a long half life emitter.

9. An image intensifier or converter as claimed in claim 8, in which said window is made of sapphire or calcium aluminate.

References Cited UNITED STATES PATENTS 3,128,408 4/1964 Goodrich et a1. 2502l3 RAL-PH G. NILSON, Primary Examiner.

MARTIN ABRAMSON, Assistant Examiner.

U.S. Cl. X.R. 250-2l3 

